In this kind of flowmeter the frequency of emission of the alternating vortices is related to the local speed of the fluid flow, and the measurement of this frequency permits the flowrate in the conduit to be obtained. The measurement of this frequency of emission is generally done by means of sensors responsive to the emission or the passage of the vortices and adapted to detect the alternating variations of local pressures resulting from their emission for producing an electrical signal related to the flowrate of the fluid.
The obstacle is generally chosen rectangular in cross section and placed diametrically across the conduit. In a large range of flowrates, the Strouhal number S=(f.multidot.d)/V
where
f is the vortex frequency,
d is the width of the obstacle transverse to the fluid flow, and
V is the fluid speed,
remains constant; in other words, the vortex frequency is closely proportional to the fluid speed.
However, it has been found that for a Reynolds number at the obstacle less than 20,000 that is for example for low flow of gas at low pressure, the average vortex frequency would be higher than the theoretical value derived from the above equation. Thus the error of the flowmeter expressed as a function of flowrate is constant for gas pressures above about 4 bars, but rises by several percent at low flowrates, causing an overestimate when the pressure of the gas being metered is very low.
This type of error is particularly disadvantageous in relation to the calibration of gas meters, generally calibrated at atmospheric pressure though they will subsequently be applied to the metering of gas under pressure.
Analysis of the flowmeter signal with a spectrum analyser has confirmed that the increase in error is not due to the appearance of parasitic oscillation, but rather is due to an inherent property of the turbulent flow.